1. Field of the Invention
The invention relates to the reading of information recorded in optical form.
Optical disks, namely disks containing information that can be read by optical systems, are now well known.
2. Description of the Prior Art
These disks work on the following principle: the information elements are most often constituted by holes etched along circular tracks separated by inter-track spaces devoid of holes. A laser focused on a small surface illuminates the tracks (and partially also the inter-track space). The disk rotates beneath the laser so that only one track at a time moves past beneath the focused beam. The beam is reflected towards a photodetector and the reflection coefficient depends on the presence or absence of a hole in the track. The passing of the holes beneath the beam thus produces a modulation of the amplitude of the reflected beam. The photodetector produces a signal modulated accordingly, representing the moving past of the holes. It is this signal that enables the reconstitution of the useful information, namely the position of the holes along each of the tracks, or the geometry of these holes.
In this technique, the plane surface of the disk between the tracks acts as a reference plane for the detection of the useful information.
It is possible to store information at high densities provided that the laser beam can be very well focused on very small holes along very narrow tracks.
This assumes notably that there are very highly precise focusing objectives devoid of aberrations and positioned with very high precision laterally to the tracks and vertically to the reference surfaces of the disk. This also assumes working with reading lasers of the shortest possible wavelength, which is not always compatible with low manufacturing costs.
It is an aim of the invention to propose a reading system that reduces the constraints related to the present reading techniques, so as to provide for a more efficient compromise between manufacturing costs (the cost of the laser focusing optical systems, the positional servo-control mechanical systems and the laser itself) and performance characteristics (information storage density, quality of the information elements restored etc.).
The central idea of the invention entails the carrying out not of a simple detection of the amplitude (light energy) of the laser beam modulated by the stored optical information elements but of a combined detection of amplitude and phase of the coherent optical wave of the modulated laser beam.
Indeed, when a wavefront of a coherent magnetic wave (a laser emits a coherent electromagnetic wave of this type) encounters an obstacle constituted in this case by an optical information element (a hole etched in a plane surface or another disturbance in the path of the wave), the wavefront is deformed and then conveys a complex information element (complex in the mathematical sense of the term, relating to amplitude and phase) directly related to the constitution of the obstacle.
The modulation of energy of the modulated beam (for example the beam reflected by the surface of an optical disk) is only a component of this complex information element. It is the easiest component to detect. The complex information, which is the amplitude and the phase of the wave at each point of the space or, again, the exact shape of the wave front modulated by the obstacle, is more abundant than the simple information on the total energy of the reflected beam. But, naturally, it is not easy to detect. Indeed, the photodetectors measure only a received energy.